A) Field of the Invention
This invention relates to a manufacturing method of gate oxidation films that is adequate for being used in manufacturing a MOS-type IC, etc., and more specifically relates to a manufacturing method of a plurality of gate oxidation films having different thickness from each other formed at one thermal oxidation process.
B) Description of the Related Art
It is well known conventionally that a plurality of MOS-type transistors having different thickness from each other are formed on a semiconductor substrate when a MOS-type IC, etc. is manufactured. In addition to that, relating to that technique, various types of methods have been suggested for forming a plurality of gate oxidation films having different thickness from each other formed at one thermal oxidation process. FIG. 16 to FIG. 18 show one example of the method of forming gate oxidation films. The entire contents of Japanese Laid-open Patent 2000-195968 are herein incorporated by reference for detail of the method of forming gate oxidation films.
In a process shown in FIG. 16, a field insulating film 2 made of a silicon oxidation film is formed on one main surface of a silicon substrate 1 by a selecting oxidation process. Thereafter, sacrificing films 3a and 3b made of silicon oxidation films are formed respectively on surfaces inside element holes 2a and 2b of the insulating film 2 by a thermal oxidation process. Thickness of the oxidation films 3a and 3b are both at about 15 nm.
Next, on the surface of the substrate, a resist layer 4 having a hole 4a corresponding to the element hole 2a is formed by a photolithography process. An ion implantation layer 5 is formed on the surface of the element hole 2a with the resist layer 4 as a mask and via the oxidation film 3a by implanting argon (Ar) ion. An accelerating voltage at this time is about 15 keV. In the ion implantation layer 5, an oxidation velocity of silicon will be increased because crystallinity of the silicon is destructed by the ion implantation.
In a process shown in FIG. 17, after removing the resist layer 4 by a chemical treatment, etc., the oxidation films 3a and 3b are removed by a hydrogen fluoride treatment. The hydrogen fluoride treatment is executed under a condition for removing an oxidation film with thickness of 30 nm when the thickness of the oxidation films 3a and 3b are 15 nm. Therefore, the insulating film 2 also becomes thinner slightly. Thereafter, a cleaning process such as RCA cleaning having etching effect is executed to the surface of the silicon substrate 1 as a proceeding process for a thermal process.
In a process shown in FIG. 18, oxidation films 6a and 6b formed of silicon oxidation films are formed on the surfaces inside the element holes 2a and 2b by the thermal process. By executing the thermal process under a condition for making thickness of the gate oxidation film 50 nm, a silicon oxidation film with thickness of 80-100 nm can be obtained as the gate oxidation film 6a because the oxidation velocity of the silicon has been accelerated near the silicon surface inside the element hole 2a by the ion implantation layer 5. Then, a first and a second MOS-type transistors respectively with the oxidation films 6a and 6b as gate insulating films are formed inside the element holes 2a and 2b by a normal silicon gate process, etc.
According to the above-described prior art, productivity is increased by forming the gate oxidation films 6a and 6b having different thickness by one thermal oxidation process, and high reliability can be achieved by avoiding pollution of the silicon surface because the surface of the silicon is covered by the sacrificing oxidation films 3a and 3b at the time of forming and removing the resist layer 4.
However, the implantation of the Ar ion is executed via the sacrificing oxidation film 3a at a low acceleration voltage such as about 15 keV, and therefore, the ion implantation layer 5 is formed in shallow region near the surface of the silicon because the range of implanting ion is about 17.1 nm. Therefore, in the process shown in FIG. 17 when the cleaning process such as the RCA cleaning having etching effect is executed as a proceeding process of the thermal oxidation process after the removal of the sacrificing oxidation films 3a and 3b, the surface part of the ion implantation layer 5 having the highest ion density is removed by the etching. When the thermal process in FIG. 18 is executed thereafter, the ion density in the ion implantation layer 5 is further lowered by out-diffusion. Therefore, oxidation efficiency in the element hole 2a will be decreased, and it makes difficult to obtain the oxidation film 6a having enough thickness compared to the oxidation film 6b. 